This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to the correction of spatial distortion in magnetic resonance images due to magnetic field inhomogeneities in disconnected regions being imaged.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
The term "stereotaxis" refers to a collection of neurosurgical techniques that apply simple geometric relationships to radiological studies thereby facilitating the treatment of certain disorders of the brain with great accuracy. Over the past decade computed tomography (CT) has been the primary mode of target localization in stereotaxic surgery. However, with the advent of the high resolution, high SNR, fast MR scanners, MR is more frequently applied to stereotaxic surgery because of its superior soft-tissue contrast.
In magnetic resonance imaging (MRI)-based stereotaxic surgery, a coordinate frame (an MR fiducial system) consisting of a set of MR-imageable fiducial rods is rigidly affixed to the head. The rods are made of a material that produces high signal intensity in MR images. The head and the MR fiducial system are then imaged, the locations of the fiducial rods are estimated from the images, and targets within the head are referenced to the MR fiducial system. In practice, however, the fiducial rods and the head cause local distortions in the main magnetic field. This, in turn, causes spatial distortion in the images, thus leading to poor target localization accuracy.
U.S. Pat. No. 5,351,006 is directed to an improved method and apparatus for correcting spatial distortion in magnetic resonance images due to magnetic field inhomogeneities in an object being imaged.
The method is practiced in vivo by establishing a map of the magnetic field, B.sub.o, at various points in an object to be imaged, thereby defining the geometric distortion due to magnetic field inhomogeneities at the various points. The B.sub.o map is obtained by applying different magnetic pulse sequences from which a measure of magnetic field inhomogeneities can be obtained. For example, gradient recalled echo (GRE) sequences or spin echo sequences can be applied so that the phase difference in two acquisitions gives a measure of the magnetic field inhomogeneity at each point.
The present invention is directed to distortion correction that specifically addresses the case where several disconnected regions are present in the imaged volume. This case includes stereotaxic frames mounted on heads.